It gets confusing when pinning them to the taskbar 
I’ll just whip up my own icons for now.
First google:
https://m16h.com/windows-icons-for-unity-3-5-and-4-0/
Seconded. It’s not entirely necessary but I would also like to be able to differenciate my Unity versions in the Taskbar by default. 
For now, I solved my problems with Jumplist Extender (seems to work fine on windows 8.1)
It’s a tool I found on google code for adding tasks to the jump menu of specific items pinned to the taskbar.
https://code.google.com/p/jumplist-extender/downloads/detail?name=JumplistExtender_v0.4.exe&can=2&q=
Now I can just get to work on unity, or mess about with any of the other versions, without having to add 3 identical icons on the taskbar.